In digital radio telephones serial bit streams of data are transmitted over-the-air. The bit streams are used to modulate a carrier. There are several types of modulation schemes used to transmit data carried by the bit stream. For example, in GSM the modulation scheme used is Gaussian Minimum Shift Keying (GMSK) whereas in CDMA systems the modulation technique used is QPSK.
GMSK is a phase modulation that converts a serial bit stream into a phase shift of a carrier wave. The function of the modulation is to convert the incoming serial bit stream into analog signals that modulate the carrier of the transmitter. In GMSK the outgoing phase shift is filtered. The Gaussian function acts as a filter, removing the sharp edges of the digital pulses. Without this filtering the required bandwidth to transmit the signal would be far greater. Even with the gaussian filter it is acknowledged that the GSM system is spectrally inefficient. The GMSK modulation does, however, provide a constant amplitude signal that is power efficient.
In existing CDMA systems a different phase modulation technique, QPSK, is chosen to provide a higher bit rate. In QPSK orthogonal signals are transmitted which double the data rate relative to MSK modulation. In QPSK modulation the outgoing phase shift is Nyquist filtered to provide root raised cosine shaped pulses that increase the spectral efficiency and reduced bit error rate by eliminating intersymbol interference. Although QPSK with root raised cosine pulse shaping is spectrally efficient allowing a high data rate and providing a low BER, it is power inefficient.
The GSM system and existing CDMA systems were designed to meet user needs considered appropriate at their conception. As schemes for third generation systems are being planned the criteria desirable in a telecommunication system that will need to provide for user's needs well into the twenty-first century are being considered.
In third generation systems it will be important that the data rates are high enough to allow the expansion of the telecommunication industry from voice into data applications to continue without reducing the power efficiency below that acceptable for a battery powered terminal. None of the existing modulation schemes allow the data rates to be high enough to support the myriad of data applications that are required without sacrificing bit error rate and/or power efficiency to an unacceptable extent.